From a commercial standpoint, vinyl chloride homopolymers and copolymers of vinyl chloride and other monomers, such as vinyl acetate, are among the most important polymers presently produced.
In the suspension or emulsion process, the polymerization reaction is conventionally carried out in an agitated reactor, from which the slurry or latex produced as a result of the polymerization reaction is stripped in the reactor at a temperature below about 65.degree. C, or sometimes transferred to a hold tank, otherwise known as a stripper. A vacuum is applied to the stripper to remove unreacted monomer, and the slurry or latex is subsequently dewatered and dried to produce dried polymer.
The difficulty is, however that this procedure as conventionally practiced, results in the production of a slurry or latex of high vinyl chloride monomer content, for example, 100-15,000 parts per million (weight/weight).
There is recent evidence that vinyl chloride monomer may be a cause of rare liver cancer, known as angiosarcoma. Note Federal Register, Vol. 39, No. 92 - Friday, May 10, 1974 - pages 16898-16900.
The removal of VCM from the slurry or latex in the stripper or other equipment suitable for condensing, compressing or otherwise capturing the VCM reduces the concentration of VCM levels in the air in working areas is requirred for safe plant operation.
An additional benefit is that the VCM captured is added to the recovered monomer pool, thereby improving polymerization efficency and economy. The added captured monomer accounts for about an additional one to four percent of the total VCM charged.
Poly(vinyl chloride) homopolymer and copolymers of vinyl chloride and other monomers are conventionally subjected to hot operations, such as milling, calendering and extruding, which release vinyl chloride monomer associated with the homopolymer or copolymer. There is therefore a need in the art for a procedure whereby there can be produced a homopolymer or copolymer of reduced vinyl chloride monomer content. This invention answers that need.
In the suspension process, vinyl chloride homopolymer and copolymers of vinyl chloride and other monomers are prepared by suspending the vinyl chloride monomer alone or in a mixture with other monomers in water by suspending agents and agitation. The polymerization is started by means of a suitable free radical generating polymerization initiator, such as lauroyl peroxide, benzoyl peroxide, diisopropylperoxydicarbonate, tertiary butylperoxypivalate, azobisisobutyronitrile, azobis-2,4-dimethylvaleronitrile, combinations of dialkylperoxydicarbonates and lauroyl peroxide, sulfonyl peroxides, and the like. Suspending agents such as methylcellulose, hydroxymethylcellulose, hydroxyethylcellulose, hydroxypropylmethylcellulose, hydrolyzed polyvinyl acetate, poly(vinyl alcohol), gelatin, methyl vinyl ether-maleic anhydride copolymers and combinations of the above are included in the reaction mixtures.
For the emulsion process, vinyl chloride homopolymer and copolymers of vinyl chloride and other monomers are prepared by conventional emulsion polymerization techniques. Free radical initiators, such as hydrogen peroxide, organic peroxides, persulfates, and redox systems are used. Surface active agents, such as alkyl sulfates, alkane sulfonates, alkylaryl sulfonates and fatty acid soaps are employed to emulsify the vinyl chloride monomer and comonomers, if any. An alternate method of dispersion resin preparation is described in U.S. Pat. No. 2,981,722 to Enk et al., granted Apr. 25, 1961.
When a copolymer of vinyl chloride and other monomers is prepared, the weight percent of the monomer, or mixtures of monomers, other than vinyl chloride can be up to thirty weight percent, based upon the total weight of the vinyl chloride and the other monomer or monomers copolymerized with the vinyl chloride. Among the monomers which can be copolymerized with the vinyl chloride are:
acrylic acid PA1 acrylonitrile PA1 n-butylacrylate PA1 diallyl maleate PA1 dibutyl maleate PA1 diethyl fumarate PA1 dimethyl itaconate PA1 ethyl acrylate PA1 ethylene PA1 isobutylene PA1 maleic anhydride PA1 methacrylic acid PA1 methacrylonitrile PA1 methyl acrylate PA1 methylvinyl ether PA1 2-ethylhexyl acrylate PA1 propylene PA1 triallyl cyanurate PA1 triallyl isocyanurate PA1 trimethylolpropane trimethacrylate PA1 vinyl acetate PA1 N-vinylcarbazole PA1 vinylidene chloride PA1 vinylisobutyl ether PA1 N-vinylpyrrolidone
Further details concerning the production of vinyl chloride homopolymer and copolymers of vinyl chloride and other monomers by suspension and emulsion polymerization are set forth in Kirk-Othmer, Encyclopedia of Chemical Technology, Second Edition, Volume 21, pages 373-379.
In accordance with this invention, vinyl chloride is removed from the slurry or latex produced as a result of the conventional application of vacuum by further processing the slurry or latex, so that vinyl chloride vapor in a further amount is removed from the slurry or latex. The uniqueness of this invention is the addition of vinyl acetate or other organic liquid to the slurry or latex, before, during or following the normal residual monomer recovery, and removing vinyl chloride from the slurry or latex by stripping techniques practiced in the art, preferably at a temperature within the range from 25.degree. C. to 125.degree. C. The result is that the slurry or latex contains significantly reduced VCM content so that subsequent processing does not constitute a health hazard in the operating work areas. The result is that dried polymer produced after dewatering is of reduced vinyl chloride monomer content, and can therefore be further processed with reduced health hazard.
In practicing this invention, a single organic liquid, including one or more solutes, if desired, or a mixture or organic liquids, including one or more solutes, if desired, can be used. Organic liquids and solutes which can be used are esters such as vinyl acetate, vinyl propionate, n-butyl acrylate, dibutyl maleate; diethyl fumarate, dimethyl itaconate, ethyl acrylate, methylacrylate, octyl acrylate, ethyl acetate, di-2-ethylhexylphthalate and di-n-octylphthalate; hydrocarbons such as pentane, heptane, white oil, cyclohexane, methycyclopentane, benzene, toluene, xylene, ethylbenzene, styrene, pentene, hexene, heptene, butadiene, isoprene; alcohols such as methyl alcohol, ethyl alcohol, isopropyl alcohol, butanol, 2-ethylhexanol, dodecanol, cetyl alcohol, ethylene glycol, propylene glycol, and butylene glycol; phenols such as phenol, di-tertiary butyl para-cresol, styrenated phenols, butylated phenols and bisphenol A; ethers such as diethyl ether, methyl vinyl ether and low molecular weight polyethers; nitrogen-containing compounds such as triethylamine, triethanolamine, aniline, dimethylaniline, morpholine, pyridine, pyrrole, acetonitrile, acrylonitrile, methacrylonitrile, acetamide, dimethylacetamide, dimethylformamide, caprolactum, N-vinylcarbazole and N-vinylpyrrolidone; nitro compounds such as nitromethane, nitroethane and nitrobenzene; halogen-containing compounds such as methylchloride, methylene chloride, chloroform, carbon tetrachloride, ethylene dichloride, ethyl chloride, vinylidene chloride, chlorobenzene, and fluorinated methanes and ethanes; carboxylic acids such as formic acid, acetic acid, acrylic acid, methacrylic acid, lauric acid, stearic acid and oleic acid; aldehydes such as propionaldehyde, butyraldehyde and benzaldehyde; ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, benzoin, benzophenone, methyl vinyl ketone and cyclohexanone; carboxylic acid anhydrides such as maleic anhydride and phthalic anhydride; acid chlorides such as acetyl chloride, benzoyl chloride and phthaloyl chloride; sulfur-containing compounds such as carbon disulfide, ethyl mercaptan, dodecyl mercaptan, dimethyl sulfoxide, dimethyl sulfone, tetramethylene sulfone, thioglycolic acid and thiophene; cyclic ethers such as dioxane, trioxane, furan, tetrahydrofuran, propylene oxide and butylene oxide; conventional diester and polymeric plasticizers for PVC such as phthalates, maleates and adipate diesters, phosphate esters and polyesters; dimethyl silicones; and liquid stabilizers for PVC such as dialkyl tin dilaurates including dibutyl tin dilaurate. In general the amount of organic solvent added to the slurry or latex will be from about 0.5 to about 20 percent by weight, based upon the weight of the slurry or latex, although smaller and larger amounts can also be used.
Those skilled in the art will understand that when the organic liquid used is of relatively low boiling point, such as vinyl acetate or methylene chloride, the liquid is essentially separated from the resin when the process of this invention is carried out. On the other hand, when the organic liquid used is relatively high boiling, such as di-2-ethylhexylphthalate, the organic liquid used essentially remains with the resin. This can be a further advantage of our invention, when the organic liquid is a conventional additive to vinyl chloride homopolymers and copolymers, as when the organic liquid is a commonly known stabilizer, lubricant, plasticizer, antioxidant, antifoam, antistatic agent, etc.
The following Examples illustrate the invention, and are not limitative thereof.